The present invention generally relates to gas-solids transport techniques and heat exchange techniques wherein a solid particulate material suitably sized for pneumatic conveyance is transported or circulated for selective contact and/or direct heat exchange with fluid or gaseous media. Loose packed bed flow and pneumatic conveyance, together with the force of gravity, are employed to transport the particulate material. The transport and heat exchange techniques are particularly useful for recovering heat from high temperature process gases such as flue gas and, in preferred heat exchange systems, the particulate material is circulated in a "figure 8" or a circular "zero loop" flow path or pattern and heat transfer between separate streams of gaseous media is provided as the particulate material contacts one gaseous stream and then the other. The gaseous streams are maintained separate by means of loose packed bed columns of particulate material formed in the flow path and used to introduce the particulate material into the gaseous streams.
The catalytic cracking of hydrocarbons has resulted in a number of prior art gas-solids transport techniques wherein the solid comprises a particulate catalyst which is circulated between a reactor cycle and a regenerator cycle in a figure 8 flow pattern. In addition to hydrocarbon cracking, similar gas-solids transport systems are employed for other chemical processes. The control of particulate material flow in such prior art systems generally includes regulation of the gravity withdrawal of the particulate material through standpipes or downcomers with the use of mechanical valves. The prior art also discloses the monitoring of pressure differentials across light and dense phases of fluidized beds for feed purposes and across valves for purposes of regulating the flow of particulate material therethrough.
The subject gas-solids transport techniques are well illustrated in the disclosed heat exchange systems, methods and apparatuses hereinafter described in detail. The heat exchange systems have been found especially advantageous in severe temperature and environment applications such as the recovery of heat from flue gases at temperatures substantially higher than 1000.degree. F. In such severe applications, the prior art heat recovery systems are substantially limited to heat wheels, regenerative devices characterized by brick arrangements, and pebble devices. These prior art devices are not of particular concern herein, and they essentially represent different classes of heat exchangers and heat exchanger techniques characterized by substantial capital investments and space requirements.